1. Technical Field of the Invention
The present invention relates to a thermal infrared solid-state image sensor having thermal isolation, particularly to a bolometer-type infrared solid-state image sensor.
2. Description of the Related Art
Conventionally, the bolometer-type infrared solid-state image sensor of this kind has adopted a structure in which diaphragm simply sandwiched a bolometer thin film with an insulating protective film in a plurality of infrared detecting elements, which consist of the diaphragm spacing above a semiconductor substrate and supported by beams. Alternatively, the image sensor had comprised an infrared absorption film on a side of the insulating protective film, to which light is made incident. Then, to obtain an infrared image, the infrared detecting elements have been arranged in two-dimensionally above the semiconductor substrate and a readout circuit corresponding to the infrared detecting elements has been formed on the semiconductor substrate.
To obtain a highly defined image by such a bolometer-type infrared solid-state image sensor, the number of infrared detecting elements above the semiconductor substrate need to be increased. However, the infrared solid-state image sensor becomes large if the number of elements is simply increased without changing the sizes of the infrared detecting elements. Therefore, reduction of the sizes of infrared detecting elements is required to achieve high definition.
Dimension reduction of the bolometer thin film is necessary for the reduction of the sizes of the infrared detecting elements. A vanadium oxide thin film is generally used as the bolometer thin film. Mr. Wada et al, after having used the vanadium oxide thin film as the bolometer thin film and inspected the relationship between the bolometer thin film dimension and noise occurred in the film, reported that the noise in the bolometer thin film had increased with the reduction of volume of the bolometer thin film (refer to SPIE Vol. 3379, p. 90, 1998). 1/f noise makes up most of the noise component, the 1/f noise is inversely proportional to the square root of the sum of free carriers (equation reported by Mr. Wada, et al), and thus the above-described noise characteristic appears. In other words, there has existed a problem that the noise occurred in the bolometer thin film increased when the size of infrared detecting element was reduced to achieve high definition.
To solve the problem, Mr. Hata and Mr. Nakagi (Japanese Patent Laid-open No. 2000-346704 publication) proposed a bolometer-type infrared detecting element whose microfabrication can be realized without increasing the noise. Description will be made for the conventional example by Mr. Hata and Mr. Nakagi using FIG. 1.
FIG. 1 is a cross-sectional structural view of the bolometer-type infrared detecting element that forms a unit pixel constituting an infrared solid-state image sensor. A plurality of the bolometer-type infrared detecting elements are formed above a semiconductor substrate 10 so as to form an array state. The infrared detecting elements are characterized by comprising a bolometer section 13 of a multi-layer structure. The bolometer section 13 is one in which a first bolometer film 13a and a second bolometer film 13b have been laminated via an insulative junction film 14, and the both bolometer films consist of vanadium oxide.
The bolometer section 13 is arranged so as to bridge over a first electrode 12a and a second electrode 12b, which are formed on a support film 11. Further, the bolometer section 13 is supported by a support leg 1B, and thermally isolated from the semiconductor substrate 10. The junction film 14 that lies between the first and the second bolometer films 13a, 13b is provided with two through holes 15. The two through holes 15 are directly positioned above the electrodes 12a, 12b respectively, and vanadium oxide is filled in the both through holes 15. The first and second bolometer films 13a, 13b are electrically connected to each other via vanadium oxide filled in the through holes 15, and the bottom surface of the first bolometer film 13a is connected to the both first and second electrodes 12a, 12b, so that the first and second bolometer films 13a, 13b are parallelly connected between the first and second electrodes 12a, 12b. In addition, as shown in FIG. 1, the bolometer-type infrared detecting element includes insulating film 16, infrared absorbing film 17, support layer 18, and wiring layer 19. Alternatively, there are cases where the first and second bolometer films 13a, 13b are connected in series. With such a lamination structure, it is possible to increase the volume of the bolometer film leaving the two-dimensional size thereof as it is, and the noise can be reduced.
As a method of increasing the volume of the bolometer film leaving the two-dimensional size thereof as it is, simply increasing the thickness of the bolometer film is an option. However, Mr. Hata and Mr. Nakagi (Japanese Patent Laid-open No. 2000-346704 publication) verified that the noise cannot be reduced by such a volume increasing method. It is understood that this is because the free carriers that cannot be effectively used increase as the film thickness becomes thicker and the increase of the sum of effective free carriers equal to the volume increase cannot be obtained. Consequently, the above-described publication adopted the structure where the thin bolometer films 13a, 13b were laminated via the insulative junction film 14 in order to obtain the increase of the sum of effective free carriers equal to the volume increase.
Generally, the bolometer film made of metal oxide such as vanadium oxide is thermally treated in a reduction atmosphere to optimize a resistance temperature coefficient, a sheet resistance, or the like. In the above-described conventional example (Japanese Patent Laid-open No. 2000-346704 publication), the bolometer films are in a laminated structure, so that there exists a problem that thermal treatment to the bolometer film of the upper layer affects the bolometer film of the lower layer and its characteristic is changed to deviate from the optimal state. Further, since there is little provability that the change amount becomes uniform between the pixels, another problem occurs that characteristic dispersion between pixels increases.
Furthermore, there exists the following problem on the point that, in the foregoing conventional example, electrical connection between the bolometer film of the upper layer and the bolometer film of the lower layer is made by the contact between the bolometer films. Specifically, although it is desirable that the bolometer films have a high resistance temperature coefficient, the resistance temperature coefficient and the resistivity are in a proportional relationship when the film material is the same. Accordingly, it is desirable that the resistivity be as high as possible from the viewpoint of the resistance temperature coefficient. However, the higher the resistivity becomes, the more difficult to obtain ohmic characteristic of the junction section. Particularly, this is even more difficult in the metal oxide such as vanadium oxide because it has stronger semiconductor characteristic in the higher resistivity. Thus, there also exists a problem that a high resistance temperature coefficient and a good electrical characteristic cannot be compatible in the conventional example.